Method and apparatus for detecting stall buffet

ABSTRACT

An aircraft instrument has been designed to indicate impending stall buffet. An angle of attack indicator is provided with a dial having two scales, one displaying angles of attack and the other displaying the maximum Mach number that may be flown for each angle of attack. A Mach number indicator is associated with the Mach scale to display the aircraft&#39;&#39;s operating Mach number. Stall buffet is indicated when the values of the maximum Mach number and the operating Mach number converge on one another. Additionally, the present instrument will also indicate the maximum angle of attack that can be flown for each operating Mach number. Stall buffet will be indicated when the values of the operating and maximum angle of attack converge. Separate warning means and aircraft control actuators can be provided so as to indicate impending stall buffet and to control the aircraft so as to remove it from approaching stall buffet.

United States Patent Daudt, Jr.

[451 Aug. 29, 1972 [54] METHOD AND APPARATUS FOR DETECTING STALL BUFFET[72] Inventor: Charles H. Daudt, Jr., PO. Box

511, Ridgefield, Conn. 06877 22 Filed: Feb. 11,1970

[21] Appl. No.: 10,407

Related US. Application Data [63] Continuation-in-part of Set, N0.567,129, Jan.

22, 1966, Pat. N0. 3,507,246.

Primary Examiner-Donald O. Woodiel Attorney-Robertson, Bryan, Parmelee &Johnson [57] ABSTRACT An aircraft instrument has been designed toindicate impending stall buffet. An angle of attack indicator isprovided with a dial having two scales, one displaying angles of attackand the other displaying the maximum Mach number that may be flown foreach angle of attack. A Mach number indicator is associated with theMach scale to display'the aircraft's operating Mach number. Stall buffetis indicated when the values of the maximum Mach number and theoperating Mach number converge on one another. Additionally, the presentinstrument will also indicate the maximum angle of attack that can beflown for each operating Mach number. Stall bufi'et will be indicatedwhen the values of the operating and maximum angle of attack [56]References Cited V converge. Separate warning means and aircraft con-UNTTED STATES PATENTS trol actuators can be provided so as to indicateimpending stall buflet and to control the aircraft so as to 3,040,5676/1962 Brody ..73/ 178 R remove it from approaching Stan buffet3,188,861 6/1965 Miller ..73/178 T 8Claims, 5 DrawingFigures SERVO SERVO CONTROL CONTROL CIRCUIT CIRCUIT 28 COMPUTER CONTROL CIRCUIT 20\ /25AN G LE OF M ACH Y ATTACK N UM BER TRANSMITTER TRANSMITTER mmtiumz m23586; 93s

SHEET 1 OF 3 3 25 BUFFET BARRIER E B u 20 O LLI z, 0 5 2 ,E VA J 0 IO C,9 2

'- 3 s E 5 o z 2.5- EXTRlAPOLqTED I MACH NUMBER Fl G. l

ANGLE OF ATTACK DEGREES LOCAL FIG. 2

INVENTOR CHARLES H. DAUDT BY Ro ey-gflnw 1 PM &

ATTORNEYS P'A'TENTEBMZ I912 SHEET 2 BF 3 ANGLE OF ATTACK DEGREES SERVOSERVO CONTROL CONTROL CIRCUIT CIRCUIT COMPUTER CONTROL CIRCUIT I 20 25ANGLE OF MACH ATTACK NUMBER TRANSMITTER TRANSMITTER FIG 4 INVENTORCHARLES H. DAUDT BY PWM/QLL&

ATTORNEYS PATENTED I972 3.686. 936- SHEET 3 0F 3 SENSOR SENSOR AMPLIAMPLI- FIER PIER (32 COMPUTER CONTROL CIRCUIT 37 ELEvATOR CONTROLACTUATOR 5 3s\ THROTTLE CONTROL F G ACTUATOR Al LERON CONTROL ACTUATOR.INVENTOR Egg $5 H. DAUDT BY J 512W) T MMM a ATTORNEYS METHOD ANDAPPARATUS FOR DETECTING STALL BUFFET BACKGROUND The present invention isa continuation in part of application, Ser. No. 567,129 filed Jan. 22,1966 now Pat. No. 3,507,246.

The present invention relates to an aircraft instrument andinstrumentdial and, in particular, to an aircraft instrument with two scales; oneindicating angles of attack and the other indicating correspondingmaximum Mach number for each angle of attack.

There have been a number of airplane accidents caused in part by theairlines philosophy of using indicated airspeed instrumentation to avoidstall or stall buffeting conditions, that is the shaking of theaircrafts flying surface controls and sometimes the whole aircraft justprior to stall. Using indicated airspeed, pilots operate their aircraftat a theoretically predetermined airspeed which incorporates for safetyan airspeed margin above stall buffet. This safety margin has beenheretofore unknown by the pilot.

Indicated airspeed, however, does not take into consideration a numberof factors which either by themselves or in combination with one anothercan combine to create stall buffeting conditions. Some of these factorsare the aircrafts weight, bank angle, pitch angle, the pilotsovercontrolling, air turbulance, G" forces and Mach number. Additionallyindicated airspeed may not fully take into account all of the variousmaneuvers that a pilot may have been forced to make while flying hisaircraft.

The problem is further complicated when the aircraft operates at higheraltitudes, i.e., 30,000 feet and above, because at these high altitudesthe indicated airspeed margins decreases. The increased criticalness ofthe higher altitude operation has heretofore been unrealisticallycompensated for by providing the pilot with flight manual, charts andgraphs whereby he may obtain at a given instant the theoreticalindicated airspeed to avoid stall buffeting conditions. The aboveprocedures have proven to be both inaccurate and impractical in that thepilot must estimate the G force on the plane at the particular instantin question for him to select the proper graph. Even if a G meter wereto be installed in the aircraft, the above method is impractical in thatthe proper graph cannot be entered and the problem solved fast enough tosatisfy the changing flying conditions.

As a consequence of the above, pilots operating jet aircraft at highaltitudes, hopefully at the safe indicated airspeed have sometimes lostcontrol of their aircraft when, in the midst of a maneuver, such asvaring pitch and bank angles in storm turbulence, the aircraft wouldunexpectedly enter stall buffet. While some aircraft recovered from thestall, some suffered severe structural damages such as losing an enginepod and some were even lost.

The airlines today generally recommend that their aircraft be operatedat 30,000 feet and below when transversing areas of suspected severeturbulence because at these altitudes the safe operating range isgreater and the pilot is less apt to lose control of his aircraft as aresult of stall buffet. Operating at such altitudes, however, has itsdisadvantages in that there is more of a chance of penetrating severeair turbulence with a corresponding increase in the probability ofairframe structural damage and of course passenger discomfort. Theinstrument of the present invention will allow an aircraft to safelyoperate well above 30,000 feet by providing the pilot withinstrumentation that will indicate impending stall buffet.

It has been found that the point at which stall buffeting will begin canbe expressed by a relationship between two factors; angle of attack andMach number. I

SUMMARY OF INVENTION An angle of attack indicator has been utilized toexpress the relationship between Mach number and angle of attack. Bycalibrating the angle of attack meter to read maximum Mach number, it ispossible for the pilot operating his airplane to, at a glance, determinefor his particular angle of attack, the maximum Mach number under whichhe must operate his plane. The relationship of angle of attack tomaximum Mach number is determined empirically for each particular typeof aircraft by operating the aircraft at a specified Mach number andincreasing the angle of attack until the plane is in stall buffet. Thisoperation is repeated for various Mach numbers. The values which arethus obtained are thereafter used to calibrate the angle of attackindicator by displaying the Mach number, at which stall buffeting willbegin, adjacent to its corresponding angle of attack.

A Mach number indicator may be incorporated into the instrument so as todisplay the aircrafts operating Mach number. This provides the pilot ofthe aircraft with the additional advantage of being able to see visuallyon one instrument the difference between the Mach number at which stallbufieting will begin and his actual operating Mach number. By coupling aproperly programmed computer to the instrument, this difference may'beutilized to operate a warning indicator to indicate impending stallbuffet or to operate aircraft controls so as to prevent the aircraftfrom approaching stall buffet.

DESCRIPTION OF THE DRAWING The accompanying drawings forming a part ofthis specification illustrate several embodiments of the invention.

FIG. 1 is a graph of angle of attack versus Mach number showing thebuffet barrier.

FIG. 2 is the face view of the instrument showing an associated angle ofattack pointer.

FIG. 3 is a face view of the aircraft instrument of FIG. 2 incorporatinga Mach member pointer.

FIG. 4 is a schematic view of the system associated with the instrumentof FIG. 3.

FIG. 5 is a schematic of the instrument of FIG. 3 with a warningindicator and control actuators.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates tothe novel dial structure which is used in conjunction with an angle ofattack indicator so as to provide at each angle of attack an indicationof the Mach number above which an aircraft should not be operated. Stallbuffet is caused by a combination of factors, some of which are theangle of bank of the aircraft, the angle of pitch of the aircraft, theweight of the aircraft, the extent of turbulence being met in thesurrounding air, the extent of overcon trolling by the pilot and thevalue of the Mach number. Of the above factors and the variouspermutations which exist amongst them, the conditions that combine tocause stall buffet have been reduced to an interrelationship between twofactors: (1) angle of attack, and

(2) Mach number. The interrelationship of these two factors is expressedin FIG. 1.

FIG. l shows a flight test calibration curve for a swept wing aircraftand is a reasonable representation of the angle of attack Mach numbercurve for various aircraft of this general type. The curve defines thelimiting Mach numbers'which can be attained without buffet for thevarious angles of attack as indicated. For

each given angle of attack, as can be seen, there exists a Mach numberat which stall buffeting is initiated. The shaded region, above thebuffet barrier, represents the stall bufiet zone.

Assuming a cruising configuration of angle of attack and 0.5 Mach(Position A in FIG. 1-), the pilot can enter stall buffet in either ofthree ways; he could increase his angle of attack to approximately 22and maintain his mach number (Point B in FIG. 1), he could increase hisMach number to approximately 0.81 and maintain his angle of attack(Point C in FIG. 1) or he could by both increasing his angle of attackand Mach number enter stall buffet, e.g., Point D in FIG. 1. Once pastthe buffet barrier he would be in stall buffet and would thereafter losecontrol of his aircraft. I

From FIG. 1 it is apparent that for each Mach number there exists anangle of attack below which a plane must fly in order to avoid stallbuffet (the area below the buffet barrier). An angle of attack indicatorhas been utilized to express this relationship by displaying the relatedMach number adjacent to the angle of attack at which stall buffet takesplace. FIG. 2 discloses an angle of attack indicator modified so as toread maximum Mach number. The dial 1 has an angle of attack scale 2arcuately arranged as shown. This type scale has been widely used inangle of attack indicating instruments. The pointer 3 is attached to theshaft 8 of the instrument and indicates relative to scale 2 angle ofattack. This dial has been made rotatable through a limited angle aboutits axis, in some cases, so that the reference marks may be adjusted toconform to special conditions which may include design feature of theaircraft carrying the instrument. The pointer is made narrower adjacentto the buffet Mach scale so that this scale can be readaccurately.

The mark 4 indicates the low speed buffet index for landingconfiguration. The mark 5 is the approach index establishing thereference for the angle of attack pointer during landing approach at theoptimum angle of attack. Marks 6 and 7 may be used for other key anglesof attack indexes such as for maximum lift to drag ratio when theaircraft is in the enroute or clean configuration, for maximum enduranceand for one or two engine inoperative enroute or clean configurationclimbout.

The Mach number scale 9 labeled Buffet Mach is I arcuately disposed withrespect to the axis of the instrument pointer shaft 8 but at less aradius than the angle of attack scale. It will be noted that the Machnumber values increase as the angle of attack values on scale 2decrease. This is true, of course, since the higher speed, higher Machnumber and resulting more separation, the less is the permissible angleof attack in order to avoid stall buffet.

By thus calibrating angle of attack meter to read related Mach numberopposite the angle of attack at which stall buffet will occur, it ispossible for the pilot operating the airplane to, at a glance, determinefor his particular operating Mach number the angle of attack at whichstall buffet will occur and under which he must operate his plane.Conversely, the pilot will also know for the angle of attack at whichhis plane is,

operating, the Mach number at which stall buffet will occur. By havingtwo scales which are read with relation to one pointer, the situation ofthe aircraft with respect to angle of attack and limiting Mach numbercan be read rapidly and reliably. This critical relationship cannot beestimated by reference to instruments conventionally provided for thepilot.

The instrumentation of FIG. 2 may be improved by including an indicatorfor displaying operating Mach number. This embodiment is shown in FIG.3. The pilot may, therefore, without the necessity of referring to aseparate Mach meter instrument, determine for his particular angle ofattack and Mach number the proximity of stall buffet. More particularlyas seen in FIG. 3, the instrument of FIG. 2 has been modified byincluding pointer 15 for displaying operating Mach number. Scale 2 inthis embodiment serves two purposes: 1. Pointer 3 in conjunction withscale 2 indicates the angle of attack at which the aircraft isoperating; and v 2. Scale 2 is correlated and adjacent to the Machnumber scale 9 so as to indicate the maximum angle of attack at whichthe aircraft may fly for that Mach number.

Scale 9 in this embodiment also serves two purposes:

1. Pointer 15 in conjunction with scale 9 indicates the Mach number atwhich the aircraft is actually operating; and

2. Scale 9 is correlated and adjacent to the angle of attack scale 2 soas to indicate the maximum Mach number that the aircraft may fly forthat angle of attack.

In the particular instance shown in FIG. 3, the aircraft is operatingwith an angle of attack of approximately 12.5 and 0.4 Mach number. Withrelation to the graph of FIG. 1, it is operating at Point E.

The instrument shown in FIG. 3 provides a visual representation of themargin that the pilot is operating below stall buffet. The pilot cantherefore by properly using the instrument avoid stall buffetingconditions. To determine impending stall buffet the pilot firstestablishes the Mach number at which the aircraft will enter stallbuffet for the angle of attack at which it is operating and also theaircrafts operating Mach number. These two values of Mach are comparedand the diflerence between the two determined. If this difference isless than a predetermined amount, e.g. 0.05 Mach, the pilot will knowthat he is approaching stall buffet and will undertake the propercorrective mea-. sures to prevent the aircraft from entering into abuffeting condition. The process of determining the difference betweenthe two Mach numbers is displayed visually by the instrument of FIG. 3.As the pointers converge on one another the aircraft is approaching thebuffet barrier.

Each pointer 3 and 15 indicates or represents two separate values.Pointer 3 represents both the aircrafts operating angle of attack(displayedon scale 2) and the maximum Mach number for that angle ofattack (displayed on scale 9). Likewise pointer 15 represents both theaircrafts operating Mach number (displayed on scale 9) and the maximumangle of attack (displayed on scale 2). The difference between eitherthe operating Mach number and maximum Mach number or the differencebetween the operating angle of attack and the maximum angle of attackmay be utilized to indicate impending stall buffet, These differencesare used herein interchangeably.

The system associated with the instrument of FIG. 3, is shown in FIG. 4.Angle of attack transmitter 20 is coupled to computer control circuit 21which in turn generates a control signal to the servo control circuit 28to control the operation of drive motor 22. Motor 22 moves the pointer 3on the angle of attack scale 2 through gear train 26. On the Mach meterside, the Mach number transmitter 25 is also coupled to the computercontrol circuit 21 which in turn generates a control signal to the servocontrol circuit 29 to control the operation of drive motor 23. Pointer15 on Mach number scale 9 is driven through gear train 27 which iscoupled to motor 23.

In operation, angle of attack transmitter 20 and Mach number transmitter25 measure respectively the angle of attack and Mach number at which theaircraft is operating and transmit a signal proportional to the measuredvalues to the computer control circuit 21. The computer control circuit21 upon receiving a signal from the angle of attack transmitter 20energizes servo control circuit 28 and motor 22 to drive pointer 3through gear train 26 to its appropriate position, on the angle ofattack scale 2. Likewise, when the computer control circuit 21 receivesa signal from Mach number transmitter 25 it energizes servo controlcircuit 29 and motor 23 to drive pointer 15 through gear train 27 to itsappropriate position on the Mach number scale 9. Computer controlcircuit 21 is adapted to move pointer For warning purposes, the signalgenerated by the computer control circuit 32 may be utilized to operatea warning indicator 33. Warning indicator 33 may be a warning light,buzzer or even a volt meter calibrated, for example, with a scale from 0to 10. As the voltage approached zero, the pilot would know that thedifference between maximum and operating Mach number was becoming smalland that stall buffet was impending. Furthermore, the warning signalcould be activated at some pre-determined minimum difference before theoperating Mach number equaled the maximum Mach number. For example, thewarning signal could be activated whenever the operating Mach number was0.05 Mach or less below the maximum Mach value for that angle of attack.

For control actuator purposes, the signal generated by the computercontrol circuit 32 may be further utilized by the computer controlcircuit to actuate the control actuators 37, 38 and 39. Controlactuators 37, 38 and 39. Control respectively, the operation of theaircrafts elevator, throttle and ailerons. When the difference betweenthe operating and maximum values approaches a preselected value then thecomputer control circuit would select the proper actuator or combinationof actuators that will safely and efficiently remove the aircraft fromapproaching stall buffet.

For example, the computer control circuit 32 could determine that anaircraft in a bank and operating at a relatively high Mach number wasapproaching stall buffet which was caused by an increase in Mach numberdue to an updraft. The control circuit computer 32 would thenselectively send a signal to the throttle actuators to reduce powerthereby reducing the aircrafts Mach number. In another example, thecomputer control circuit 32 could determine that an aircraft in aclimbing turn and operating at a relatively low Mach number wasapproaching stall buffet due to an-increase in the aircrafts angle ofattack caused by the aircrafts entry into a climbing turn. The computercontrol circuit 32 would then selectively send a signal to the aileronactuators to level the wings (zero bank angle), send a signal to thethrottle actuators to increase power to decrease the angle of attack andsend a signal to the elevator actuators to hold the decreasing angle ofatover the nonlinear Mach number scale 9.

The present apparatus can be adapted as shown in FIG. 5 to generatewarning and actuator signals. The apparatus again consists of anindicator such as in FIGS. 3 and 4, pointer 3 indicates both angle ofattack and the corresponding maximum Mach number for that angle ofattack. Pointer 15 indicates the planes operating Mach number as well asthe corresponding maximum angle of attack. for that Mach number. Theoperating angle of attack and its corresponding maximum Mach number aresensed by sensor circuitry 35, amplified by amplifier 30 and transmittedto computer control circuit 32. The operating Mach number and itscorresponding maximum angle of attack are sensed by sensor circuit 36,amplified by amplifier 31 and transmitted to computer control circuit32. Computer control circuit 32 is adapted to receive the signalsgenerated from both sensors, compare the two, determine the differencebetween the two, and generate a third signal which is proportional tothe difference thereof. Computer control circuit 32 is of the type whichis known to persons skilled in computer art.

tack caused by the throttle action to increase power.

I claim:

1. An aircraft instrument for indicating impending stall buffet, saidinstrument including:

a. an angle of attack indicator with a scale calibrated in degrees ofangle of attack and with a pointer associated with said angle of attackscale so as to display the planes angle of attack;

. a Mach number scale adjacent and related to the angle of attack scaleso as to represent for the corresponding angle of attack the Mach numberat which stall buffeting will begin; and

c. a Mach meter pointer associated with said Mach number scale todisplay the value of the Mach number of the aircraft thereon.

. The aircraft instrument of claim 1 further includa control circuit tosense the difference between the Mach number at which stall buffetingwill be initiated and the aircrafts operating Mach number and togenerate a control signal proportional to the difference between the twoMach numbers; and

e. a warning indicator responsive to the control signal.

3. The aircraft instrument as claimed in claim 2 wherein the warningindicator is calibrated to visually display the difference between theaircrafts operating Mach number and the Mach number at which stallbuffeting will begin.

4. Apparatus for indicating impending stall bufiet comprising:

a. A dial with one scale calibrated in degrees of angle of attack and asecond scale adjacent to the first scale calibrated so as to display themaximum Mach number for that attack before which stall buffeting isinitiated;

b. an angle of attack indicator in conjunction with the said dialwhereby both the angle of attack and the corresponding maximum Machnumber for that angle of attack are displayed; and

c. a Mach number indicator arranged in conjunction with said dial so asto display both the Mach number at which the aircraft is operating andthe corresponding maximum angle of attack for that Mach number.

5. Apparatus of claim 4 further including:

d. control circuit to sense the difference between the maximum andoperating Mach number and to generate a signal proportional to thisdifference; and

e. warning means responsive to the said generated stall buffet, saidinstrument including:

a. Mach meter with a scale calibrated in Mach numbers and with a pointerassociated with said Mach number scale so as to display the aircraftsoperating Mach number;

b. An angle of attack scale adjacent and related to the Mach numberscale so as to represent opposite the aircrafts operating thecorresponding angle of attack at which stall buffeting will occur; and

c. a angle of attack indicator associated with said angle of attackscale to display the value of the angle of attack at which the aircraftis operating.

7. The aircraft instrument of claim 6 further including:

(1. a control circuit to sense the difierence between the angle ofattack at which stall buffet will occur and the aircrafts operatingangle of attack and to generate a control signal that is proportional tothe difference between the two angles of attack; and

e. a warning indicator responsive to the signal generated by the controlcircuit.

8. The aircraft instrument of claim 7 wherein the warning indicatorvisually displays the difference between the aircrafts operating angleof attack and the angle of attack at which stall buffeting occurs.

1. An aircraft instrument for indicating impending stall buffet, saidinstrument including: a. an angle of attack indicator with a scalecalibrated in degrees of angle of attack and with a pointer associatedwith said angle of attack scale so as to display the plane''s angle ofattack; b. a Mach number scale adjacent and related to the angle ofattack scale so as to represent for the corresponding angle of attackthe Mach number at which stall buffeting will begin; and c. a Mach meterpointer associated with said Mach number scale to display the value ofthe Mach number of the aircraft thereon.
 2. The aircraft instrument ofclaim 1 further including: d. a control circuit to sense the differencebetween the Mach number at which stall buffeting will be initiated andthe aircraft''s operating Mach number and to generate a control signalproportional to the difference between the two Mach numbers; and e. awarning indicator responsive to the control signal.
 3. The aircraftinstrument as claimed in claim 2 wherein the warning indicator iscalibrated to visually display the difference between the aircraft''soperating Mach number and the Mach number at which stall buffeting willbegin.
 4. Apparatus for indicating impending stall buffet comprising: a.A dial with one scale calibrated in degrees of angle of attack and asecond scale adjacent to the first scale calibrated so as to display themaximum Mach number for that attack before which stall buffeting isinitiated; b. an angle of attack indicator in conjunction with the saiddial whereby both the angle of attack and the corresponding maximum Machnumber for that angle of attack are displayed; and c. a Mach numberindicator arranged in conjunction with said dial so as to display boththe Mach number at which the aircraft is operating and the correspondingmaximum angle of attack for that Mach number.
 5. Apparatus of claim 4further including: d. control circuit to sense the difference betweenthe maximum and operating Mach number and to generate a signalproportional to this difference; and e. warning means responsive to thesaid generated signal to display this difference.
 6. An aircraftinstrument for indicating impending stall buffet, said instrumentincluding: a. Mach meter with a scale calibrated in Mach numbers andwith a pointer associated with said Mach number scale so as to displaythe aircraft''s operating Mach number; b. An angle of attack scaleadjacent and related to the Mach number scale so as to representopposite the aircraft''s operating the corresponding angle of attack atwhich stall buffeting will occur; and c. a angle of attack indicatorassociated with said angle of attack scale to display the value of theangle of attack at which the aircraft is operating.
 7. The aircraftinstrument of claim 6 further including: d. a control circuit to sensethe difference between the angle of attack at which stall buffet willoccur and the aircraft''s operating angle of attack and to generate acontrol signal that is proportional to the difference between the twoangles of attack; and e. a warning indicator responsive to the signalgenerated by the control circuit.
 8. The aircraft instrument of claim 7wherein the warning indicator visually displays the difference betweenthe aircraft''s operating angle of attack and the angle of attack atwhich stall buffeting occurs.